This invention relates to a polypropylene film, multi-layered polypropylene film, a self-tacky film, a multi-layered self-tacky film and multi-layered polypropylene film for a high-temperature sterilization packing. More particularly, this invention relates to a film having excellent transparency, impact resistance at low temperature and heat sealing property and also having excellent property of little whitening property upon bending and property of preventing the deterioration of transparency after heating; to a multi-layered film having excellent transparency, impact resistance at low temperature and heat sealing property and also having excellent property of little whitening property upon bending and property of preventing the deterioration of transparency after heating; to a self-tacky film having excellent transparency, impact resistance at low temperature and heat sealing property; a self-tacky multi-layered film having excellent transparency, impact resistance at low temperature and heat sealing property; and a multi-layered polypropylene film having little deterioration of transparency after a high-temperature sterilizing treatment, having excellent heat resistance, impact resistance at low temperature and tight sealing of the content and being suitable for a high-temperature sterilizing treatment packing for food.
Polypropylene has mechanical, optical and thermal characteristics and is excellent in a packing aptitude and, therefore, it has been widely utilized in the use for packing of food and in the use for daily necessities.
With regard to a polypropylene material for polypropylene film, a propylene homopolymer, a copolymer or a terpolymer of propylene with xcex1-olefin and a composition comprising a propylene homopolymer and a copolymer of propylene with ethylene (the so-called block copolymer) are used and each of them is used for a packing use meeting with each characteristic.
However, the conventional known polypropylene film is insufficient in terms of a balance of transparency, impact resistance at low temperature, heat sealing property, self-tackiness, little whitening property upon bending and effect of preventing the lowering of transparency after heating. Thus, there are problems that, in the film of a propylene homopolymer, impact resistance at low temperature, heat sealing property and self tackiness are poor and, in addition, little whitening property upon bending is insufficient; in the film using a copolymer or a terpolymer of propylene with xcex1-olefin, transparency and heat sealing property are excellent but impact resistance at low temperature, self-tackiness, little wh itening property upon bending and an effect of preventing the lowering of transparency after heating are poor; and, in the film using a polypropylene composition comprising crystalline polypropylene which does not satisfy the specific limiting viscosity ratio, weight ratio, etc. and a copolymer of propylene with xcex1-olefin, impact resistance at low temperature is excellent but transparency, heat sealing property, self-tackiness and little whitening property upon bending are poor and, in addition, f ish eye is apt to be generated.
In such a film using the conveational polypropylene, it is difficult to satisfy all of the requirements for transparency, impact resistance at low temperature, heat sealing property, self-tackiness, little whitening property upon bending and an effect of preventing the lowering of transparency after heating treatment.
In addition, as a result of development of distribution mechanism and improvement in freeze transportation technique, there has been a quick increase in food product where cooked food or processed food is packed, sterilized with steam at high temperature and distributed at ambient temperature or in a warmed or frozen state. As to a film used for sterilization by steam (hereinafter, referred to as xe2x80x9cfilm for retort pouchxe2x80x9d), the film which is made of polyolefin has been used and, with an object of prevention of generation of deformation (wrinkles, shrinking, etc.) and fusion of the film at the steam sterilization temperature (120xc2x0 C. or higher but lower than 140xc2x0 C.) or prevention of breakage of the film for retort pouch during the distribution at low temperature, a film using a crystalline propylene-ethylene block copolymer having excellent heat resistance and impact resistance at low temperature is used as polyolefin. However, the film using a crystalline propylene-ethylene block copolymerhas a poor transparency and there is a disadvantage that the content therein cannot be seen. When transparency is needed for the packing material with an object of seeing the content, a polyolefin film in which a crystalline propylene random copolymer is compounded with a non-crystalline ethylene-propylene copolymer is used. However, in a film where a crystalline propylene random copolymer is compounded with a non-crystalline ethylene-propylene copolymer, there are disadvantages that the transparency lowers after steam sterilization and that fusion occurs during the steam sterilization at high temperature. Recently, film for retort pouch having a transparency by the use of a linear low-density polyethylene is developed but, even in the said film, there is a problem that the film is deformed or fused upon steam sterilization. Under such circumstances, with an object of making the lowering of transparency after a high-temperature sterilization treatment little and of improving the heat resistance and the impact resistance at low temperature, there is a proposal for a compounded film where (i) the first layer comprising polypropylene or an ethylene-propylene random copolymer, (ii) the second layer comprising an ethylene-propylene block copolymer or a blend of an ethylene-propylene block copolymer with a polyolefin polymer well compatible with rubber components in the said block copolymer and (iii) the third layer comprising polypropylene or an ethylene-propylene random copolymer are layered (Japanese Patent Laid-Open No. 3950/1987).
In the meanwhile, the present applicant already filed a Japanese Patent Application No. 181,141/1996 (International Laid-Open WO 97/19135) for a multi-layered film comprising a layer using a propylene block copolymer where a limiting viscosity is within a specific range and a layer using polypropylene having well-balanced heat resistance, impact resistance at low temperature, tear strength, transparency, appearance and heat sealing property.
The present invention is to provide a film having excellent transparency, impact resistance at low temperature, heat sealingproperty and self-tackiness and also having little whitening property upon bending and an effect of preventing the lowering of transparency after heating treatment and also to provide a multi-layered film having excellent transparency, impact resistance at low temperature, heat sealing property and self-tackiness and also having little whitening property upon bending and an effect of preventing the lowering of transparency after heating treatment. Further, from the above-mentioned characteristics, the present invention is to provide a polypropylene film and multi-layered film which can be appropriately used in various uses such as a film for retort pouch to be sterilized by heating, a film for medical use, a film for layered bag package and a film for the manufacture of boxes.
The present inventors have carried out an intensive investigation for preparing a film which satisfies all of transparency, impact resistance at low temperature, heat sealing property, self-tackiness, little whitening property upon bending and an effect of preventing the lowering of transparency after heating treatment. As a result, it has been found that a polypropylene composition comprising crystalline polypropylene and a copolymer of propylene with xcex1-olefin in which a limiting viscosity of the copolymer, a limiting viscosity ratio of both crystalline polypropylene and copolymer components and a product of the limiting viscosity ratio with the ratio by weight of the both components are within a predetermined range is made into a film, the resulting film has excellent transparency, impact resistance at low temperature, heating sealing property and self-tackiness and having little whitening property upon bending and an effect of preventing the lowering of transparency after heating treatment. They have further carried out an intensive investigation to prepare a film for retort pouch in which a lowering of transparency after subjecting to a high-temperature sterilization treatment is little and heat resistance, impact resistance at low temperature and a tight sealing of the content are improved. As a result, it has been found that a multi-layered film comprising three layers wherein a propylene block copolymer composition where a limiting viscosity is within a specific range is used as a core layer while a propylene polymer where the melting point of crystals is from 145xc2x0 C. to 165xc2x0 C. is used as a skin layer is able to solve the disadvantage of the above-mentioned film for retort pouch and, based upon such findings, the present invention has been accomplished.
The present invention is as follows.
(1) A film using a polypropylene composition comprising crystalline polypropylene and a copolymer of propylene with xcex1-olefin in which a limiting viscosity of the said copolymer [xcex7]RC is not more than 6.5 dl/g; the ratio ([xcex7]RC/[xcex7]PP) of the limiting viscosity of the said polymer [xcex7]RC to the limiting viscosity of the said crystalline polypropylene [xcex7]PP is from 0.6 to 1.2; and the product, i.e. ([xcex7]RC/[xcex7]PP)xc3x97(WPP/WRC), of the ratio by weight (WPP/WRC) of the said crystalline polypropylene to the said copolymer is within a range of from 0.2 to 4.5 where the weight of the crystalline propylene is WPP and that of the said copolymer is WRC with the said limiting viscosity ratio ([xcex7]RC/[xcex7]PP).
(2) The film according to the above (1), wherein the crystalline polypropylene is a propylene-xcex1-olefin copolymer containing 90% by weight or more but less than 98.5% by weight or more than 99.5% by weight but less than 100% by weight of propylene.
(3) The film according to the above (1), wherein the copolymer is a copolymer of propylene with xcex1-olefin containing from 25 to 55% by weight of (X-olefin.
(4) The film according to the above (1), wherein the crystalline polypropylene is a propylene-ethylene copolymer containing 90% by weight or more but less than 98.5% by weight or more than 99.5% by weight but less than 100% by weight of propylene.
(5) The film according to the above (1), wherein the copolymer is a copolymer of propylene with ethylene containing from 25 to 45% by weight of ethylene.
(6) A multi-layered film having at least one layer of a film layer using the polypropylene composition mentioned in the above (1).
(7) A self-tacky multi-layered film having at least one layer of the film mentioned in the above (1) at the skin layer.
(8) A polypropylene multi-layered film for a high-temperature sterilization package consisting of a three-layered structure in which a film layer using the propylene composition mentioned in (1) is a core layer and a film comprising a propylene homopolymer having a crystal melting point of from 160xc2x0 C. to 165xc2x0 C. or a propylene-xcex1-olefin copolymer having a crystal melting point of from 145xc2x0 C. to 160xc2x0 C. as both skin layers.
(9) The polypropylene multi-layered film for a high-temperature sterilization package according to the above (8), wherein the thickness of the core layer of the polypropylene multi-layered film for a high-temperature sterilization package mentioned in the above (8) is from 20% to 80% of the total thickness of the said polypropylene multi-layerd film.
(10) The polypropylene multi-layered film for a high-temperature sterilization package according to the above (8), wherein the melt flow rate: MFR/film (at 230xc2x0 C.) of the polypropylene multi-layered film for a high-temperature sterilization package is within a range of from 0.5 g to 5.0 g/10 minutes.
(11) The polypropylene multi-layered film for a high-temperature sterilization package according to the above (8) f wherein the differencebetweenthehazevalue (HAZE/after) of the polypropylene multi-layered film after subjecting to a steam high-temperature sterilization treatment of 120xc2x0 C. or higher but lower than 140xc2x0 C. to the haze value (HAZE/before) of the polypropylene multi-layered film before subjecting to the steam high-temperature sterilization treatment is less than 5%.